A depth filter retains particles throughout the depth of a filtration media. Various media can be used to construct a depth filter, one of them being a non-woven media of melt blown or spun bond filaments. A depth filter may have multiple layers (or zones), with the layer having the largest pore size usually forming an upstream layer and the layer having the smallest pore size forming a downstream layer. This is in contrast to surface filters, alternatively called screen filters, which retain particles primarily by size exclusion on or near an upstream separation layer rather than throughout the depth of the filter. A surface filter may provide some depth filtration for particles below its rated absolute particle size, but the amount of depth filtration is limited by the surface filter's lack of thickness and the desire to make any layers behind the upstream separation layer as permeable as possible. A depth filter may be distinguished from a surface filter by way of the depth filter's substantial thickness, which is typically at least 5 mm and more often at least 10 mm. A depth filter is also typically provided in a configuration that provides a smooth inner and outer peripheral surface to maximize its volume whereas a surface filter is typically folded or pleated so as to maximize its surface area.
A cartridge filter is a removable or replaceable filter element designed to be placed in a housing. Some cartridge filters may be cleaned, but they are typically discarded at the end of their useful life. A depth filter cartridge may be rated according to its dirt holding capacity (DHC), which is measured in grams of solid particles that the filter can hold before plugging. The useful life of a cartridge is measured as the time the filter element can be operated under specified conditions before reaching a specified maximum pressure drop across the depth filter cartridge. The useful life of a cartridge may be limited by its DHC or by its mechanical ability to withstand the applied pressure as it becomes loaded with particles. Other rating criteria include the efficiency of the filter in removing particles of a specified size and the clean water pressure drop of the filter. For example, a removal efficiency rating may be specified as 90% removal of particles down to a specified micron size or as “absolute” (meaning 99%) removal of particles down to a specified micron size.
U.S. Pat. No. 6,986,427, issued on Jan. 17, 2006 to Aune et al., describes a melt blown non-woven media useful for a depth filter element. The media is made by directing a plurality of melt blown filaments at the side of a conical end of a tubular structure. The tubular structure rotates on a spinning mandrel. The tubular structure grows in length as material is added to its conical end while the tubular structure is drawn out of the filament spray area along the length of the mandrel. Different filaments are directed at different portions of the cone, and the filaments may vary in one or more characteristics along the length of the cone. This produces concentric annular zones in the tubular element with a corresponding variation in the one or more characteristics. One or more other melt blown filaments may be applied across the length of the cone to add filaments that extend through the depth of the element, crossing multiple zones, to strengthen the media.
U.S. Pat. No. 6,938,781, which shares a common priority application with U.S. Pat. No. 6,986,427, describes a non-woven depth filter cartridge that includes a cylindrical mass of essentially continuous melt-blown polymer filaments and an essentially continuous traversing melt blown polymer filament extending through the mass. The cylindrical mass has a depth dimension, a longitudinal dimension, and a circumferential dimension. The filaments of the cylindrical mass are generally oriented in the longitudinal and circumferential dimensions and form a plurality of concentric zones. The traversing filament extends in the longitudinal dimension through a substantial portion of a length of the cylindrical mass while extending around the cylindrical mass in the circumferential dimension and extending radially in the depth dimension through substantially an entire thickness of two or more zones of the cylindrical mass.
Polypropylene depth filter cartridges made according to the patents described above are sold by GE Water and Process Technologies in association with the Z.PLEX trade mark. These cartridges have inside diameters of about 1 inch and outside diameters of about 2.5 or 2.75 inches. They are used in a number of water filtration applications.